1. Field of the Invention
The present invention relates to transistors and particularly to transistors utilizing field plates.
2. Description of the Related Art
Improvements in the manufacturing of AlGaN/GaN semiconductor materials have helped advance the development of AlGaN/GaN transistors, such as high electron mobility transistors (HEMTs) for high frequency, high temperature and high power applications. AlGaN/GaN has large bandgaps, high peak and saturation electron velocity values [B. Gelmont, K. Kim and M. Shur, Monte Carlo Simulation of Electron Transport in Gallium Nitride, J. Appl. Phys. 74, (1993), pp. 1818-1821]. AlGaN/GaN HEMTs can also have 2DEG sheet densities in excess of 1013 cm−2 and relatively high electron mobility (up to 2019 cm2/Vs) [R. Gaska, et al., Electron Transport in AlGaN-GaN Heterostructures Grown on 6H-SiC Substrates, Appl. Phys. Lett. 72, (1998), pp. 707-709]. These characteristics allow AlGaN/GaN HEMTs to provide very high voltage and high power operation at RF, microwave and millimeter wave frequencies.
AlGaN/GaN HEMTs have been grown on sapphire substrates and have shown a power density of 4.6 W/mm and a total power of 7.6 W [Y. F. Wu et al., GaN-Based FETs for Microwave Power Amplification, IEICE Trans. Electron. E-82-C, (1999). pp. 1895-1905]. More recently, AlGaN/GaN HEMTs grown on SiC have shown a power density of 9.8 W/mm at 8 GHz [Y. F. Wu, et al., Very-High Power Density AlGaN/GaN HEMTs, IEEE Trans. Electron. Dev. 48, (2001), pp. 586-590] and a total output power of 22.9 W at 9 GHz [M. Micovic, et al., AlGaN/GaN Heterojunction Field Effect Transistors Grown by Nitrogen Plasma Assisted Molecular Beam Epitaxy, IEEE Trans. Electron. Dev. 48, (2001), pp. 591-596].
U.S. Pat. No. 5,192,987 to Khan et al. discloses GaN/AlGaN based HEMTs grown on a buffer and a substrate. Other AlGaN/GaN HEMTs and field effect transistors (FETs) have been described by Gaska et al., High-Temperature Performance of AlGaN/GaN HFET's on SiC Substrates, IEEE Electron Device Letters, 18, (1997), pp. 492-494; and Wu et al. “High Al-content AlGaN/GaN HEMTs With Very High Performance”, IEDM-1999 Digest, pp. 925-927, Washington D.C., December 1999. Some of these devices have shown a gain-bandwidth product (fT) as high as 100 gigahertz (Lu et al. “AlGaN/GaN HEMTs on SiC With Over 100 GHz ft and Low Microwave Noise”, IEEE Transactions on Electron Devices, Vol. 48, No. 3, March 2001, pp. 581-585) and high power densities up to 10 W/mm at X-band (Wu et al., “Bias-dependent Performance of High-Power AlGaN/GaN HEMTs”, IEDM-2001, Washington D.C., Dec. 2-6, 2001) and Wu et al., High Al-Content AlGaN/GaN MODFETs for Ultrahigh Performance, IEEE Electron Device Letters 19, (1998), pp. 50-53].
Electron trapping and the resulting difference between DC and RF characteristics have been a limiting factor in the performance of these devices. Silicon nitride (SiN) passivation has been successfully employed to alleviate this trapping problem resulting in high performance devices with power densities over 10 W/mm at 10 Ghz. For example, U.S. Pat. No. 6,586,781, which is incorporated herein by reference in its entirety, discloses methods and structures for reducing the trapping effect in GaN-based transistors. However, due to the high electric fields existing in these structures, charge trapping is still an issue.
Field plates have been used to enhance the performance of GaN-based HEMTs at microwave frequencies [See S Kamalkar and U. K. Mishra, Very High Voltage AlGaN/GaN High Electron Mobility Transistors Using a Field Plate Deposited on a Stepped Insulator, Solid State Electronics 45, (2001), pp. 1645-1662]. These approaches, however, have involved a field plate connected to the gate of the transistor with the field plate on top of the drain side of the channel. This can result in a significant field plate to drain capacitance and the field plate being connected to the gate adds additional gate-to-drain capacitance (Cgd) to the device. This can not only reduce gain, but can also cause instability due to poorer input-output isolation.